It is a well known fact that contact, and separation, of dissimilar materials can result in net electrical charging of each of the materials. Such triboelectric, or frictional, electrostatic charging can occur on dielectric, as well as electrically conductive materials. Other electrostatic charging operators, of the same or opposite polarity, can simultaneously effect the resultant net charge on a given body of material, or portion thereof, especially when the body is a heterogeneous combination of conductive and dielectric materials.
Triboelectric charge polarity on a body can be of either sign, and is a function of the type of dissimilar materials involved in the frictional contact. Charge rate, or charging current, is a function of material area, material type and the frequency and velocity of contacts between the dissimilar materials.
FIG. 1 depicts a typical dielectric 1 mounted on a conductive material 8, for instance metal. In this case, particles 2 of a differing construction are approaching and impacting on both the dielectric and metal. As the particles collide with the dielectric and metal surfaces and then separate from the contact, charges (4 or 5) are deposited on the impacted surfaces and the exiting particles 3 are themselves charged at the opposite polarity. This illustration could be represented by flight of an aircraft through precipitation, such as snow or rain, or even through masses of atmospheric suspended particulate matter like ice crystals or dust. Similar charging can be occasioned by simply rubbing dissimilar objects together, or as the result of flow of a fluid over a surface or through a pipe. Charges of both polarities are indicated in FIG. 1 (4 and 5), but contact between two materials of given compositions can be expected to generate only one set of polarities between the objects.
Some of the criteria that have been shown to influence stored electrostatic energy and potentials are: atmospheric or other fluid pressures; surrounding electric fields, especially when other charge centers or areas exist, and; such variables as relative humidity, surface contaminants, sharp points and so on. For instance corona and surface plasma breakdown potential thresholds approximately halve with each halving of atmospheric pressure. Also, dry air breaks down electrically at much higher potentials than air at higher relative humidities.
Several undesireable effects can result from electrostatic charging. When electric energy is stored by conductive material, or on dielectric surfaces, there can be a potential for electric shocks ranging from unpleasant to lethal in extreme cases. The electrical energy can ignite flammables. Sufficiently severe stored charges can puncture dielectrics, especially when they overlay or otherwise cover electrical conductors that are at a different electrical potential. If the charges penetrate electronic systems or devices they can be destructive. Finally, electromagnetic interference is created by surface streamer currents 6, corona discharges and dielectric punctures or other arcing 7.
There are no known means of preventing triboelectric charging during frictional contact between dissimilar materials. Therefore, to avoid the undesireable effects of triboelectric charging, means must be provided to discharge the resultant electrostatic charges.
An earthed conductive body will drain such electrostatic charges, and no net stored charge will result. An isolated conductive body will store the charges until corona threshold is reached, then, for a given charging rate, corona current will reach an equilibrium with the charging current. Because space charge impedes corona currents the stored energy on such an isolated conductive body is a direct function of charging current. When the charging source changes rate, or ceases to operate, the equilibrium time constant is quite short, typically ranging from a few nanoseconds to a few seconds, depending upon the stored energy levels and several other variables.
An isolated dielectric surface, on the other hand, will store triboelectric energy until the escape velocity of the generated ions is reached, the breakdown potential of surrounding fluids is achieved or the stored charge recombines with incidental ions of the opposite polarity. If the dielectric is part of a more complex structure, some or all of which is more conductive, and at a lower electrical potential than the stored charge on the dielectric, other mechanisms are likely to operate. The most common are surface streamer currents of the ions to the conductive structure. If the dielectric area is sufficiently large and conductive structure is at or near the reverse surface of the dielectric, or embedded in the dielectric, the dielectric strength may be exceeded with a resultant arc through the dielectric. This is commonly known as pinholing. Once pinholing occurs subsequent arcing pulses will occur through the pinhole at much lower potentials. Both streamer currents and arcing pulses are energetic producers of electromagnetic interference.
It has been demonstrated that dielectrics can store significant charges for hours even days. With the relatively rapid decay of stored energy from adjacent conductive bodies, the result can be very large potential differences between the dielectric surface stored charge and the conductive sections.
If the nearby conductive materials are charged at the same polarity, to near the same potential or at greater potentials than the dielectric surface, even larger stored charge energies can occur on the dielectric surface than if it were an isolated body, due to the shielding effect of the surrounding electric field. If the conductive structure charging rate is rapidly reduced, even more energetic energy releases can occur from the dielectric surface, since it is potential difference that generates streamer currents and arcing.
Current technology for reducing stored electrostatic energy on dielectrics relies on conductive coating of the dielectric surface that is subjected to charging. Such coating will distribute the electric charges uniformly over the coatings, or when bonded to conductive structure, or earthed, equalize or drain the charges.
Reasonable optical or radio frequency transparency can be obtained with current state-of-the-art conductive coatings. However, they are not always satisfactory for a number of reasons. On many dielectric materials adhesion of the coatings, at best, is difficult to achieve. Since the coatings are of necessity on the impacted triboelectrically charged, or otherwise charged, surface, maintenance of the coating and bonding integrity is frequently a source of difficulty. Erosion of the coatings by particle impact is a well known problem. Abrasion can also destroy part, or all, of the coatings or bonding provisions. Overcoating of the conductive material for decorative, erosion control or other reasons is difficult or sometimes impractical. Maintenance of coatings and bonding requires technical expertise that frequently is not available at user and service levels.
Failures of conductive coatings and electrical bonding can result in an increase of electromagnetic interference, compared to that of untreated dielectrics. For this and other reasons, such as cost, conductive coating of dielectrics may not be used, even though their benefits and desirability may be recognized.